Presence or occupancy detection is used in systems that automatically take action when an object of interest, e.g. a person, is present in a region of interest, e.g. a surveillance area. For example, a door, e.g. a sliding door, may be automatically opened when people are standing in front of it, lights may be switched on when a person enters a room or is in the vicinity of the lights, lights may be switched off when no-one is in a room, or an alarm may be triggered when an intrusion is detected.
Several methods of presence detection are known in the art. For example, pressure sensitive sensors or inductive loops may be integrated in a floor in order to detect the presence of persons or vehicles. For heat generating objects, e.g. particularly for persons and/or animals such as pets, detection may involve sensors for detecting infrared radiation emitted by these objects. Infrared detection may be carried out using compact and affordable technology, and has the advantage over other techniques, such as visual image recognition, acoustic sensing or ultrasonic detection, that warm-blooded living beings, such as humans, emit thermal radiation having a characteristic spectral distribution, e.g. a peak around 9.5 μm for humans, and having considerable power, e.g. around 100 W for humans. Since infrared radiation is emitted by these warm-blooded living beings, detection does not require external lighting. Furthermore, many materials which are opaque or only allow limited transmission of light in the visual spectrum are transparent for infrared radiation.
As an example, in state of the art devices for presence or occupancy detection, passive infrared (PIR) sensors are commonly used to detect moving heat generating objects in a surveillance area. Such a PIR sensor transforms infrared energy, e.g. heat radiation, into an electrical signal, e.g. a voltage. The term passive in this instance means that the PIR sensor does not emit an infrared beam but merely passively accepts incoming infrared radiation. PIR sensors for detecting persons may have a wavelength sensitivity peak tuned to around 10 μm, e.g. close to the 9.5 μm, the peak wavelength of infrared radiation emitted by humans. Such a PIR sensor device for occupancy detection is disclosed in U.S. Pat. No. 4,318,089. In this document, a prior art PIR sensor device for presence detection may comprise a pair of spaced apart infrared radiation sensing elements in an enclosure, such as a three-pin metal header package for semiconductor devices, e.g. a TO-5 package. The enclosure may further feature a transparent window in order to limit the radiation transmitted through the window into the enclosure to a suitable wavelength range, e.g. between 5 μm and 15 μm or between 7 μm and 14 μm. Such a transparent window may for example be manufactured from an appropriate material such as germanium, silicon or polyethylene.
In a typical prior art PIR sensor device, the pair of infrared radiation sensing elements may be pyroelectric elements, connected in a voltage bucking configuration, for example connected in anti-phase series, e.g. by an electrically connected pair of matched poles of both elements. Pyroelectric elements have a differential response; a temperature change induces a temporary voltage change over the element which will dissipate due to leakage current at constant temperature. However, a differential readout arrangement over two elements may additionally cancel out signals caused by vibration, ambient temperature changes or field-wide illumination, e.g. by sunlight. The enclosure comprising the pyroelectric elements may further comprise a sensitive field-effect transistor (FET) in order to read out the voltage over the pair of sensitive elements. The two sensitive elements in anti-phase series may for example be grounded on one terminal, and on the other terminal be connected to the gate of the FET and connected to a pull-down resistor.
The prior art sensor devices further typically comprise a focusing element such as a Fresnel lens or a multi-faceted parabolic mirror, in order to project infrared radiation emitted by an object that generates heat, e.g. a person, onto the sensing elements. This focusing element is designed such that radiation emitted by a heat-generating object moving across the surveillance area, e.g. crossing the field of view of the detection device, is projected onto the sensing elements in an alternating manner, i.e. the element on which this radiation is concentrated switches repeatedly. Thus, an alternating current is generated on the output of the FET, which may be further amplified.
The advantage of coupling the elements in a voltage bucking configuration, e.g. in anti-phase series, is that the sensor device becomes insensitive to the environment temperature. However, as the sensing elements never exhibit quite the same characteristics, offsets may arise which have to be filtered out of the system by creating a floating reference level based on an averaging low-pass filter.
In a digital embodiment of this prior art device, further filtering may condition the signal, e.g. to reduce aliasing, before sampling it with an analog-to-digital converter (ADC), usually at a low sampling rate, for example less than 10 Hz, e.g. 5 Hz. The sampling rate is typically quite low due to a low signal-to-noise ratio (SNR), e.g. a SNR of about 2. In the digital domain, the peaks of the filtered signal may be detected, which will trigger an event for a preset duration if it reached a certain level, for example a peak may trigger the switching on of a light or the opening of a door and the restarting of a timer which will switch off the light or close the door after a preset delay, which is usually user-controllable.
Similar analog circuits are known in the art which implement the same or a similar function. In both analog and digital devices, the timer delay and the sensitivity of the device may be controlled by altering a setting.
However, such PIR-based detection method may have disadvantages caused by the inherent differentiating behavior of the design. Such devices only get triggered when moving objects are detected in the surveillance area. For example, when persons stay longer motionless than a timer in a PIR-based sensor device for light switching allows, they might be surprised by the switching off of the light.
In U.S. Pat. No. 4,849,737, another PIR-based detector is disclosed. This prior art sensor is adapted for mechanically scanning a space, e.g. by arranging the PIR-based detector on a rotating disc. Thus, a person which stays substantially motionless with respect to his surroundings can be observed by this prior art PIR-based detector because the motion of the sensor establishes a relative movement between the person and the detector. However, the detection efficiency of such PIR-based sensors may still depend on the relative speed of the detector and the person being detected.
Furthermore, PIR devices as known in the art often require a complex design, e.g. carefully designed Fresnel lenses, to be able to provide an indication of direction of movement of a detected object, e.g. direction of walking of a person. Automatic sliding doors that use PIR sensors to detect persons are therefore typically not arranged at the side-walls of hallways because of the many false alarms that may be triggered by people just passing by.
Furthermore, designing a PIR-based device that is capable of providing an indication of the number of detected objects in a surveillance area, e.g. how many persons are present in a scene, poses further complications. A rudimentary indication of the number of persons present, e.g. distinguishing between a single or multiple persons, would for example be useful in efficient person specific life-style monitoring devices, e.g. for monitoring applications in elderly care in situations where more elderly share the same living spaces.